It is known that aromatic isocyanates may be converted into primary amines by acid hydrolysis. This reaction is only partially complete, however, since the amine obtained from hydrolysis reacts with unreacted isocyanates to form the corresponding urea. This secondary reaction cannot be suppressed by using an excess of a strong mineral acid. A comparatively recent example is found in Japanese Pat. No. 55007-827.
It is also known that aromatic isocyanates may be converted into aromatic amines by basic hydrolysis. The conditions for this hydrolysis, mentioned for two particular heterocyclic isocyanic acid esters in H. John, J. Prakt. Chemie, 130, 314 et seq (1931) and H. John, J. Prakt. Chemie, 130, 323 et seq (1931), are, however, completely unsuitable for the conversion of isocyanate prepolymers into the corresponding polyamines.
Processes by which carbamate salts obtained by the reaction of isocyanate compounds with aqueous bases may be converted into the free amino compounds by acidolytic decomposition with mineral acids or strong organic acids are described in German Offenlegungsschriften Nos. 2,948,419 and 3,039,600. One disadvantage of these processes is that they inevitably give rise to, for example, mineral acid salts, the removal of which renders the said processes less economical.
A process for the preparation of specific primary aromatic amines containing polyalkylene glycol ether segments is described in German Auslegeschrift No. 1,270,046. In this process, the reaction products of aromatic di- or triisocyanates with polyalkylene glycol ethers and/or polyalkylene glycol thioethers, preferably those with molecular weights of from 400 to 4,000, are reacted with secondary or tertiary carbinols and the reaction product is subsequently subjected to decomposition by heat in an inert solvent (optionally in the presence of acid catalysts). The disadvantage of this process, however, is that the thermal cleavage of urethanes is accompanied by the formation of combustible, readily-volatile alkenes which are explosive when mixed with air, necessitating that special safety measures be taken.
German Auslegeschrift No. 1,694,152 relates to the preparation of prepolymers having at least two amino end groups by the reaction of hydrazine, aminophenyl ethylamine or other diamines with an isocyanate prepolymer obtained from a polyether polyol and polyisocyanate (NCO:NH ratio=1:1.5 to 1:5). Any unreacted amine must be carefully removed by a separate process step, however, since it both powerfully catalyzes the reaction with polyisocyanates (thereby shortening the time available for processing) and itself enters into reactions.
Another possible method of synthesizing polyamines containing urethane groups is described in French Pat. No. 1,415,317. Isocyanate prepolymers containing urethane groups are converted into N-formyl derivatives by reaction with formic acid, and these derivatives are saponified to the aromatic amines with amino end groups. The reaction of isocyanate prepolymers with sulfamic acid according to German Auslegeschrift No. 1,155,907 also leads to compounds with amino end groups. Then, relatively high molecular weight aliphatic prepolymers containing secondary and primary amino groups are obtained according to German Auslegeschrift No. 1,215,373 by the reaction of relatively high molecular weight hydroxyl compounds with ammonia in the presence of catalysts under pressure at high temperatures, or according to U.S. Pat. No. 3,044,989 by the reaction of relatively high molecular weight polyhydroxyl compounds with acrylonitrile followed by catalytic hydrogenation. Relatively high molecular weight compounds containing terminal amino groups and urethane groups are also obtained according to German Auslegeschrift No. 2,546,536 or U.S. Pat. No. 3,865,791, by the reaction of isocyanate prepolymers with hydroxyl-containing enamines, aldimines or ketimines followed by hydrolysis. The main advantage of these methods is that, for example, although the aromatic amine released by hydrolysis reacts, for example, with the ketimine-isocyanate adduct to form urea, that hydrolysis, in most cases, proceeds very slowly.
Another possibility for synthesizing aromatic polyamines containing urethane and ether groups lies in ring opening, such as takes place in the reaction of isatoic acid anhydride and diols. Polyamines of this kind have been described, for example, in U.S. Pat. No. 4,180,644 and German Auslegeschriften Nos. 2,619,840; 2,648,774; 2,648,825; and 2,019,432. One disadvantage of this method, however, for many purposes, is the low reactivity of aromatic ester amines obtained by such a method.
The reaction of nitroaryl isocyanates with polyols followed by reduction to amines is also known (U.S. Pat. No. 2,888,439), but the main disadvantage of this method is the high cost of the reduction step.
It has now surprisingly been found that specific aromatic polyamines containing urethane and/or urea and/or biuret and/or isocyanurate groups and preferably also ether groups (or other groups, e.g., acetal, thioether, dimethyl siloxane or polybutadiene groups) are obtained when certain ureas described in more detail below are treated with a base, preferably in equivalent or slightly excess quantities, based on the quantity of urea groups present.
It is surprising that the basic hydrolysis of the urea groups, substituted according to the invention, proceeds so selectively on urea partial structures substituted with aliphatic or cycloaliphatic groups, that any urethane and/or biuret and/or allophanate and/or isocyanurate groups present at the same time are not split. This would not have been expected from the published state of the art. It is also surprising that the hydrolysis of monomeric low molecular weight monoureas, which is known, may also be directly applied to high molecular weight polyurea compounds.
It is advantageous that in contrast to known methods of hydrolysis of isocyanate compounds by the preliminary reaction of the isocyanate compound with secondary amines, which leads quantitatively to urea compounds, no chain lengthening, by way of an isocyanate-water reaction, occurs in this case. Furthermore, the N',N'-dialkyl urea derivatives are relatively stable derivatives of the isocyanate prepolymers which are themselves, in many cases, not stable in storage, so that these derivatives may subsequently be converted into the desired polyamines at any time after their preparation.